A porous PTFE sheet exhibit a specific properties of passing gases such as water vapor therethrough, while not permeating water, so that the sheet is excellent in moisture permeable waterproof property, by which a low humidity can be kept inside even in rain and in sports. Therefore, a porous PTFE sheet has been widely used as fabrics materials of shoes, wear and the like. In addition, a porous PTFE sheet has been widely used also as various filter materials, since a porous PTFE is excellent in heat resistance and chemical resistance, and the pore size thereof is adjustable.
On the other hand, a porous PTFE sheet is not sufficient, in compressive resistance, mechanical strength and abrasion resistance due to the structural issue that a porous PTFE sheet is made of a fluorinated resin and is porous. Hence, technology for improving the strength of a porous PTFE sheet has been studied.
For example, Patent Documents 1 to 9 disclose a porous PTFE sheet of which pores are filled with a filler, and the production methods thereof.
However, the porous PTFE sheets described in the Patent Documents are produced by blending a PTFE powder with a filler powder, and preliminarily-molding the mixture, and then expanding it. As a result, the sheets have a structure in which filler powders are adsorbed on the pores of the porous PTFE sheet, whereby it is difficult to say that the inherent defects of porous PTFE sheet are sufficiently improved.
Additionally, a sheet made of ultra-high molecular weight polyethylene has been known as a porous sheet with wear resistance. However, it is difficult to make the sheet thin, since the sheet is produced by cutting a sintered body. In addition, the sheet is made of polyethylene and thus lacks in compressive resistance and heat resistance.
Patent Document 10 discloses a composite material produced by compressing a thermoplastic resin fiber with heating to produce a porous material, impregnating the porous material with a solution of another thermoplastic resin, and then cooling it. The material is excellent in wear resistance and the like and also has high-strength and is excellent in heat resistance; however, the material does not apparently show permeability that is an inherent characteristic of porous PTFE sheet, since the pores are filled with the resin.
Patent Document 11 discloses a sheet that is improved in both of moisture permeability and wear resistance. However, the sheet is produced by a complicated process of forming a porous polyurethane main layer on a substrate, and then forming a surface layer on the main layer. In addition, it is difficult in such a production process to control the condition for maintaining moisture permeability. Furthermore, the compressibility and heat resistance cannot be improved in the sheet, since the pores of the porous layer are maintained as they are.